Thermionic cathode and method of manufacturing the same

ABSTRACT

A THORIUM FILM THERMIONIC CATHODE IN ELECTRON DISCHARGE DEVICES AND METHOD OF MANUFACTURING THE SAME WHEREIN A HIGH MELTING METAL CARRIER, SUCH AS TUNGSTEN OR TUNGSTEN ALLOY, CONTAMINATED WITH THORIUM OXIDE IS PROVIDED WITH A COATING CONTAINING AN INNER LAYER OF TUNGSTEN CARBIDE AND AN OUTER LAYER OF A HIGH MELTING METAL HAVING MIGRATINAL PROPERTIES AND FUNCTIONING AS AN EMISSION BASE FOR THORIUM, SUCH AS OSMIUM OR RHENIUM.

US. Cl. 117-217 11 Claims ABSTRACT OF THE DISCLOSURE A thorium filmthermionic cathode in electron discharge devices and method ofmanufacturing the same wherein a high melting metal carrier, such astungsten or tungsten alloy, contaminated with thorium oxide is providedwith a coating containing an inner layer of tungsten carbide and anouter layer of a high melting metal having migrational properties andfunctioning as an emission base for thorium, such as osmium or rhenium.

This invention relates to a thorium film thermionic cathode in electrondischarge devices and to a method of manufacturing the same; moreparticularly the invention relates to thorium film thermionic cathodeshaving a high melting metal carrier contaminated with thorium oxide andprovided with a layer of a material yielding maxi mum stabilization ofemission properties and minimum effective work functions for the cathodeand to the method of manufacturing the same.

R is generally well known that a reduction of the work function of puretungsten thermionic cathodes can be achieved by the inclusion oraddition of a relatively small amount of thorium oxide in the tungstenbase metal. This improvement of emission is generally attributed to thereduction of thorium oxide to metallic thorium during high temperatureannealing. The metallic thorium forms a surface coating (generallymonatomic) on the tungsten carrier metal. Such thorium-film cathodes, oras they are generally referred to thoriatedtungsten cathodes, have alower work function than pure tungsten cathodes and/or pure thoriumcathodes. These type of thorium films must be produced through thereduction of thorium oxide added to a tungsten cathode and the film mustbe maintained on the tungsten cathode during the operation thereof.

Further, it is generally known that the above described thoriatedtungsten cathode can be improved by converting the outer surface of thetungsten into tungsten carbide. A tungsten carbide coated thoriatedtungsten cathode has a more favorable reducing effect on workingtemperatures than thorium oxide. Conversion of tungsten to tungstencarbide is generally achieved, for example, by carburization of thethoriated tungsten cathode base at elevated temperatures in a reducinggas atmosphere containing a vaporized hydrocarbon. This is generallyknown as the carburization process. General- 1y, to 40% of the outercross-sectional area of the thoriated tungsten cathode base is convertedto tungsten carbide.

Another method of reducing the work function of a thoriatedhigh-emitting cathode is also known. This method consists oftransferring normally existing hexagonal structured tungsten carbidecrystals into modified cubic body-centered tungsten carbide crystals bysubjecting the same to diffusion-annealing conditions at substantiallyelevated temperatures in the presence of foreign metals or metalloids ofthe Groups IB, VII-A "United States Patent 0 3,697,321 Patented Oct. 10,1972 or VIII-A from the Periodic Chart of Atoms, such as silver,manganese or palladium.

A common disadvantageous feature of the aforesaid known thoriatedtungsten cathodes is that the carbide layers, which function as thereduction zones, forms an emission base for the cathode on the surfaceof the cathode so that a relatively wide dispersion of the emissionproperties of the cathode is attained.

Accordingly, it is an object of the present invention to provideimproved thoriated tungsten thermionic cathodes and a method ofmanufacturing the same eliminating the aforesaid problems.

It is another object of the present invention to provide a thoriatedtungsten thermionic cathode having a minimal effective working functionand a maximum stabilization of emission properties and a method ofmanufacturing the same.

It is yet a further object of the present invention to provide athoriated tungsten thermionic cathode having an inner coating oftungsten carbide and an outer coating of a high melting metal havingmigrating properties capable of functioning as an emission base forthorium and a method of manufacturing the same.

Other features, objects and advantages of the present invention willbecome more apparent with the teachings of the principles of the saidinvention in connection with the disclosure of the preferred embodimentsthereof in the specification and claims.

The present invention provides a thoriated-tungsten (or thorium film)thermionic cathode for utilization in electron discharge devices, suchas transmitter tubes. The thoriated tungsten cathode structure isprovided with an inner layer of tungsten carbide and an outer layer of ahigh melting metal having migrating properties for thorium andfunctioning as an emission base for thorium while having relativelyhigher work function than tungsten. The thoriated-tungsten cathode ofthe present invention provides effective separation of the reductionzone of the cathode from the emission zone thereof and thereby attainsall desirable advantages of dispenser cathodes.

A preferred method of manufacturing the thoriatedtungsten cathode of theinstant invention consists of providing a thoriated-tungsten basestructure (i.e. the tungsten wire having incorporated therein a fewparts per hundred of thorium oxide), and subjecting the same to theheretofore explained carburization process for a period of timesuificient to convert an outer portion of the base structure to tungstencarbide thereby forming a substantially uniform coating surrounding thebase structure. This carburized base structure is then provided with afurther or outer substantially uniform coating of a high melting metal,which has migrational properties. The high melting metal coating on theouter layer of the carburized cathode structure provides a base for themigration of thorium thereon at working temperatures of the cathode sothat deposition of thorium by diffusion through this outer layer issufficiently and continuously assured. Further, this emission base(outer layer) is continuously retained throughout the entire life orworking time of the particular cathode and thus provides an effectiveseparation of the reduction zone (tungsten carbide) from the emissionzone (such emission base or outer layer is formed of high melting metalshaving a relatively higher work function than tungsten and havingmigrational properties for thorium). In other words, the outer layer hasa thickness sufficient to provide effective separation between thereduction zone and the emission zone and allow controlled subsequentdiffusion of thorium therethrough. Preferably, this thickness is aboutThe high melting metal utilized to form the outer coating-or. emissionbase of the thoriated-tungsten cathode of the present invention, in pureform, has a higher work function than tungsten. Preferably, this highmelting metal is selected from this group and most preferably consistingof osmium, rhenium and mixtures thereof since these metals combine withthorium film to yield a particularly low etfective work function incathode structures.

The method of applying the outer layer, which functions as the emissionbase of the thoriated-tungsten cathode structure, can be selected fromelectrolytical or non-electrolytical chemical-reduction deposition froma suitable solution containing the high melting metal. Once the outer orsecond layer is deposited or formed on the first or inner layer oftungsten carbide surrounding the base thoriated-tungsten cathodestructure, the cathode is ready for installation and use in electrondischarge devices.

Thus, comparing the preesnt thoriated-tungsten cathode having a tungstencarbide layer and methods of manufacturing the same ,to the aforesaidprior art thoriatedtungsten cathodes having a tungsten carbide layercomposed of cubic body-centered crystals thereof (achieved bydiffusion-annealing in the presence of foreign metals or metalloids fromthe Groups IB, VII-A or VIII-A) it will readily be appreciated that nostructural conversion (i.e. crystal structure re-arrangement) of thecarbide layer in the cathode of the present invention need be effectedby high temperature annealing in order to activate the formed cathodes.

carburization of the metal carrier (i.e. tungsten) has an adverse effecton the mechanical strength, particularly the breaking strength of suchmetal. It has now been found thatwthe: mechanical strength of a carriermetal can be materially increased by utilizing a tungsten-rhenium alloyas the carrier metal. Preferably, the tungsten-rhenium alloy containsabout to 25% by weight of rhenium. This increase in mechanical strengthis further increased by utilizing an outer layer composed of rhenium.Rhenium counteracts embrittlement of cathode structures to such a degreethat, among other things, practically no carbide is formed on thecathode structure. Thus, the carrier metal for the thermionic cathode ofthe present invention is composed of a relatively high melting metalselected from the group consisting of tungsten and tungsten-rheniumalloy. As will be appreciated, the carrier metal is contaminated with(or contains relatively small amounts of) thorium oxide.

Typical production methods that have been found eX- ceptionally suitablein the practice of the present invention are set forthin the followingtables:

TABLE I (a) Mechanically producing a cathode structure from tungstenwire having a 1.5% thorium oxide content;

(1)) mounting the cathode structure on K-carriers;

(c) carburizing the cathode structure by pre-annealing under high vacuumconditions at 1700" C. for about minutes and then annealing in a benzolatmosphere at about 1620" C. for about 5 minutes to achieve a car-'burized layer of W C in a thickness of about ,um.;

(d) galvanically coating the resultant structure with a layer of osmiumin a thickness of about (LS/1111.;

(e) annealing the resultant structure under high vacuum conditions at 1600, C. for about 10 minutes; and

(f) installing the finished cathode structure into the intended tube.

TABLE II (a) Mechanically producing a cathode structure from atungsten-rhenium alloy containing about 24% (by weight) rhenium andhaving a 1.5 thorium-oxide content;

(b) mounting the cathode structure on K-carriers;

(c) carburizing the cathode structure by pre-annealing innitrogen-hydrogen atmosphere containing 10% hydrogen gas at 1900 C. forabout 1 minuteand then annealing in a nitrogen-hydrogen (:10 ratio)atmosphere having about 1.6% benzol vapor therein at about 1750 C. forabout 1 minute to achieve a carburized layer of W C in a thickness ofabout 20 ,um.;

(d) applying a layer of vaporized rhenium by cathode atomizers in athickness of about 0.7 ,um.;

(e) annealing the resultant structure under relatively high vacuumconditions at 1500 C. for about 10 minutes; and

(f) installing the finished cathode structure into the intended tube.

The manner in which the thorium film thermionic cathode of the instantinvention can be produced will become more apparent to those versed inthe art by reference to the following examples, which are intended onlyto be illustrative and not limiting the scope of the invention in anyway.

EXAMPLE I A commercial wire of tungsten was obtained in blank etchedform that had about 0.7% to 1.8% by weight of thorium oxide incorporatedtherein. This wire was mechanically shaped into a desired cathode form,subjected to inserted stress relieving annealing and mounted on anappropriate cathode carrier.

The cathode structure was then subjected to pre-annealing underrelatively high vacuum conditions at temperatures ranging from about1650 C. to 0 C. for a pe: riod of time ranging from about 5 to 15minutes. Thereafter the cathode structure was carburized by annealingthe same in a benzol vapor atmosphere (or some other hydrocarbonatmosphere) at temperatures in the range of about 1580 C. to 1670" C.for a period of time ranging from 3 to 8 minutes. The carburizationresults in a tungsten carbide layer thickness of about 25 ,um.

An osmium coating of about 0.5 urn. thickness was then applied by wellknown galvanic separation methods. The entire structure was thenannealed under. relatively high vacuum conditions at temperatures in therange of about 1550 C. to 1650 C. for a period of time ranging fromabout 5 to 15 minutes. The cathode was then mounted in an appropriateelectron discharge tube and produced excellent results as outlinedhereinbefore.

EXA'MPLE II A commercial wire of tungsten-rhenium alloy (containing 24%of rhenium) having about 0.7 to 2.0% by weight of thorium oxideincorporated therein was obtained to produce a carburized thoriumtungsten cathode with increased mechanical strength and increased shockresistance in accordance with the principles of the instant invention.

The thoriated tungsten-rhenium alloy wire was first deformed bymechanical manipulation into a suitable cathode shape and then mountedon a suitable cathode carrier.

The cathode structure was then subjected to a relatively short cleansingannealing in a 10% hydrogen-90% nitrogen atmosphere at about 1850 C. to1950" C. temperature for a period of time ranging from 30 seconds toabout 2 minutes. The cathode structure was then carburized in the samehydrogen-nitrogen atmosphere (10:90 ratio), to which was added about1.6% by weight of benzol vapor at temperatures in the range of about1700 C. to 1800 C. for a period of time ranging from 30 seconds to about2 minutes. The carburization procedure produced a layer of tungstencarbide on the cathode structure having a thickness of about 20 um.

Thereafter a rhenium coating of about. 0.7 pm. thickness was applied bymeans of .cathode. atomization. The entire structure was then annealedunder relatively high vacuum conditions at temperatures of about 1550 C.to 1650 C. for a period of time ranging from about 5 to 15 minutes.

The cathode was then mounted in a electron discharge device andexhibited the excellent properties described hereinbefore.

As will be appreciated, the various steps set forth in the aboveexamples may be modified and interchanged between the various examplesgiven.

It will be understood that modifications and variations of the abovedescribed preferred embodiments of the principles of the presentinvention may be effected without departing from the spirit or scope ofthe novel concepts set forth in the invention.

We claim:

1. A thoriated-tungsten thermionic cathode for electron dischargedevices comprising a thoriated-tungsten cathode base structure, an innerlayer on said base structure defining a reduction zone composed of atungsten carbide substantially surrounding said cathode base structureand an outer layer on said inner layer defining a support or carrierlayer for the emission zone composed of a relatively high melting metalhaving a relatively higher work function than tungsten and havingmigrational properties for thorium selected from the group consistingessentially of osmium and rhenium, said outer layer substantiallyuniformly surrounding said inner layer.

2. A thoriated-tungsten thermionic cathode for electron dischargedevices comprising a thoriated-tungsten cathode base structure composedessentially of a high melting metal selected from the group consistingessentially of tungsten and a tungsten-rhenium alloy containing about to25% rhenium, an inner layer on said base structure defining a reductionzone composed of a tungsten carbide substantially surrounding saidcathode base structure and an outer layer on said inner layer defining asupport or carrier layer for the emission zone composed of rhenium, saidouter layer substantially uniformly sur rounding said inner layer.

3. A thoriated-tungsten thermionic cathode for electron dischargedevices comprising a thoriated-tungsten cathode base structure, an innerlayer on said base structure defining a reduction zone composed of atungsten carbide substantially surrounding said cathode base structureand an outer layer on said inner layer defining a support or carrierlayer for the emission zone composed of a relatively high melting metalhaving a relatively higher work function than tungsten and havingmigrational properties for thorium, said outer layer substantiallyuniformly surrounding said inner layer.

4. A thoriated-tungsten thermionic cathode as defined in 6 claim 3wherein the outer layer is composed of osmium.

5. A thoriated-tungsten thermionic cathode as defined in claim 4 whereinthe thickness of the outer layer is about 0.5 am.

6. A thoriated-tungsten thermionic cathode as defined in claim 3 whereinthe outer layer is compose-d of rhenium.

7. A thoriated-tungsten thermionic cathode as defined in claim 6 whereinthe thickness of the outer layer is about 0.7 pm.

8. A method of manufacturing a thermionic cathode for electronicdischarge devices consisting essentially of the steps of: (1) providinga thoriated-tungsten cathode base structure, (2) subjecting said cathodebase structure to a carburization process for a period of time,sufiicient to convert an outer portion of said cathode base structureinto an inner layer which defines a reduction zone composed of atungsten carbide substantially surrounding said cathode base structure,and (3) coating said inner layer with an outer layer, which defines asupport or carrier layer for the emission zone, composed of a relativelyhigh melting metal having a relatively higher work function thantungsten and having migrational properties for thorium, said outer layersubstantially uniformly surrounding said inner layer.

9. A method of manufacturing a thermionic cathode as defined in claim 8wherein the step (3) is effected through non-electrolytic chemicalreduction deposition.

10. A method of manufacturing a thermionic cathode as defined in claim 8wherein the step (3) is effected through electrolytic deposition.

11. A method of manufacturing a thermionic cathode as defined in claim 8wherein the step (3) comprises coating rhenium by cathode atomization.

References Cited UNITED STATES PATENTS 2,204,391 6/1940 Allen 29-1952,497,109 2/1950 Williams 117-217 3,373,307 3/1968 Zalm et a1. 313-3113,488,549 1/1970 Amra 313-311 ALFRED L. LEAVITT, Primary Examiner C. K.WEIFFENBACH, Assistant Examiner US. Cl. X.R.

